Study On The Corrosion Behavior Of Oil Pipeline Steels In Carbon Dioxide And Hydrogen Sulfide Solution

With the continuous exploit of oil and gas in last decades, the technology of pipeline transportation developed rapidly in China. Corrosion of the pipeline deteriorates in most oilfields because of harm form high-sulfur, high-water and variety of corrosive medium, which makes factors for corrosion complicated. Many researches in corrosion under a single medium have been done in China, however, there is little study on the stress corrosion cracking and hydrogen permeation behavior of 15# steel under the H₂S, CO₂, Cl^- etc. coexisting conditions. Therefore, in order to ensure the security and reliable operation of pipeline, it is significant to do some research in the rule and mechanism of stress corrosion cracking and hydrogen permeation behavior of 15# steel under the H₂S, CO₂, Cl^- etc.Electrochemical, slow strain rate tensile test(SSRT) and the hydrogen permeation technology were used to study the stress corrosion cracking and hydrogen permeation behavior of 15# steel under the H₂S, CO₂, Cl^- etc. coexisting conditions.Experiment in electrochemical behavior of 15# steel shows (by the polarization curves) that there is state of active dissolution but active-passive transition. Comparing with under NaCl^-CO₂-H₂S solution, the diameter of high-frequency arc in electrochemical impedance spectroscopy is shorter, the corrosion current density is bigger and the corrosive rate is faster under NaCl^-CO₂ solution. So is NaCl^-CO₂-H₂S solution to NaCl^-H₂S solution. Moreover, hydrogen penetration test shows that hydrogen concentration is closely related to HIC. The higher hydrogen concentration penetrates into the metal, the greater probability of materials hydrogen-induced cracking occur with. The reason is the poisonous effect of H₂S and the stronger corrosion inhibition degree of Cl^- to CO₂ (than to H₂S), which cause the behavior of hydrogen permeation and hydrogen bubble phenomenon of 15# steel in NaCl^-H₂S solution. Also for this reason, P∞(the steady state permeation current )and the hydrogen embrittlement sensitivity are the largest. SSRT shows that both of the brittleness coefficient and stress corrosion cracking sensitivity increase while H₂S is added but decrease while CO₂ is added. At free corrosion potential, the unit extension and reduction of area dwindled away in NaCl, NaCl^-CO₂, NaCl^-CO₂-H₂S and NaCl^-H₂S solution. But the brittleness and stress corrosion crack sensitivity increase gradually under the same conditions. The results of the experiment are consistent with electrochemical experiments and hydrogen penetration test.